With increases in track densities of Hard Disk Drives (xe2x80x9cHDDsxe2x80x9d), vibration sensitivity has become an acute problem. Present generations of HDDs at densities of 10,000 tracks per inch (xe2x80x9cTPIxe2x80x9d) already exhibit poor settleout and/or track-follow characteristics when subject to external vibrations. In computer servers or arrays, where multiple disk drives are mounted in a frame, vibrations from one drive may substantially affect neighboring drives. At least three types of external vibration sources are known: 1) periodic with relatively constant peak amplitude arising from spindle imbalance; 2) non-periodic arising from reaction torque generated by an actuator seek motion; and 3) random vibrations resulting from other unpredictable external events. For example, CD-ROMs are known to produce vibrations which are periodic with gradually changing periodicity.
Conventional servo systems can reduce the track-follow error by about 20 to 30 dB using the basic servo loop error rejection property, but this level of error rejection is no longer sufficient to make HDDs immune from adverse vibration effects. This problem is expected to get worse with the accelerated growth of track densities in future products.
Present generation 2.5xe2x80x3 and 3.5xe2x80x3 direct access storage devices (xe2x80x9cDASDsxe2x80x9d) are designed to operate in both portable and desk-top/server environments. In portable computers, the 2.5xe2x80x3 DASDs are subject to harsh non-operating conditions, and moderate operating conditions. On the other hand, desk-top 3.5xe2x80x3 DASDs are not subject to as much non-operating shock. However, desk-top/server DASDs often operate in the presence of multiple vibration sources associated with other drives in a s ingle computer frame.
Mechanical components such as spindle motor assemblies are not perfectly mass balanced, and may produce harmonic vibrations during operation. Harmonic vibrations produce both a linear and a rotational oscillatory motion of the entire system. At the 10 kTPI design point, a rotational oscillatory motion of a track with respect to the actuator pivot of about 0.01 thousandth of an inch (0.25 micrometer) corresponds to 10% of the track pitch. When not compensated, a track-follow error of 10% of track pitch can be detrimental to a drive""s soft and hard error rate performance.
An HDD with a rotary actuator system is highly sensitive to rotational vibrations of the base plate. Using special shock and vibration isolation mount designs, the rotational oscillatory components due to internal spindle forcing can be minimized as disclosed in U.S. Pat. No. 5,400,196. A mount design optimized for intern al spindle vibration decoupling as in U.S. Pat. No. 5,400,196 is still susceptible to external vibrations. By deploying isolation mounts along a polygon satisfying a particular set of criteria (as taught by Japanese Patent No. 2,565,637) the externally generated vibration can also be decoupled. However, it is often a challenge to convince customers to implement special mount designs to compensate for manufacturing imperfections of HDDs.
As discussed above, present track density is about 10 kTPI, and is expected to grow exponentially with time. Positioning the read/write elements over small tracks is therefore a major challenge, especially in the presence of vibrations. Servo systems are used for positioning, and perform three critical tasks: 1) they move the head to the vicinity of a target in a minimum time using a velocity servo under seek mode; 2) they position the head on the target track with minimum settleout time using a position controller without an integrating element in it; and 3) the servo system enters the track-follow mode with a proportional-integral-derivative type (xe2x80x9cPIDxe2x80x9d) position controller.
Magnetically recordable tracks are set to move at a constant tangential velocity during operation of an HDD by the spindle motor system. Due to bearing imperfection as well as mechanical vibration of the HDD platform, the tracks may tend to move in the radial direction as well. The read/write (xe2x80x9cR/Wxe2x80x9d) elements are transported primarily along the radial direction of a disk platter thus providing an ability to follow the radial movement of the tracks. The position error signal (xe2x80x9cPESxe2x80x9d) generated from the servo sectors of a disk by the read head represents the relative error between the two mechanically moving objects, i.e., the track and the R/W head. The PES contains repeatable and non-repeatable components. The repeatable component can further be classified into short term repeatable and long term repeatable categories. Feedback or feedforward solutions take advantage of the repeatable property of the PES to formulate innovative servo solutions. A feedback (U.S. Pat. No. 5,608,586) or feedforward (U.S. Pat. No. 4,536,809) solution can be considered to compensate for repeatable track-follow error components. When a repeatable run out component frequency is known a-priori, a digital servo algorithm can be designed and embedded in a product servo microcode to suppress the position error component at the vibration frequency. The run out at this frequency can result from either a physical shift of a disk with respect to its axis of rotation or from self vibration created by spindle motor mass imbalance. As described in U.S. Pat. No. 5,608,586, a robust servo solution, including optimum initial condition setup to operate a digital filter, solves the disk shift problem with minimum settleout time penalty. A disk shift due to shock can be considered long term repeatable as it is affected by the occasional shock.
However, when a frequency of cross vibration is unknown, such as that associated with another HDD in the same frame, a servo algorithm according to U.S. Pat. No. 5,608,586 cannot be designed and embedded in the drive prior to shipment. This problem must be addressed in any effective cross vibration solution. Further, any solution should be implemented without major computational requirements since the cost constraints of HDDs usually preclude the use of a dedicated digital signal processor (xe2x80x9cDSPxe2x80x9d) to perform complex or long arithmetic functions.
What is required, therefore, are techniques to compensate for vibrations encountered in disk drives, which are adaptive to the frequencies of the vibrations, and which can be implemented without using complex computing hardware.
The present invention solves the periodic, cross vibration problem by implementing a two step algorithm which first detects the dominant cross vibration frequency, and then configures an optimal servo feedback solution. The information gathered during the detection process is used to configure a digital peak filter. The filter coefficients are tuned to compensate for the detected periodic vibration induced error. The servo algorithm uses a frequently updated initial condition vector to minimize the settleout time encountered due to the transient dynamics of the digital filter. Advantageously, no trigonometric computations, such as FFTs, are invoked in one aspect of the present invention, during the determination of the cross vibration frequency. Hence a low cost implementation in a microprocessor-based or specialized chip-based HDD servo control system is provided by the present invention.
In that regard, the present invention relates, in one aspect, to techniques for reducing the effect of a vibration on a position signal in a servo system of a data storage device. The servo system controls the position of an access element with respect to a medium mounted for movement in the data storage device. The technique involves detecting, during operation of the data storage device, a frequency of the vibration. An exemplary detection filter scans the position signal across a range of frequencies, and, at each respective scanned frequency, records an amplitude associated therewith. The recorded amplitudes are then examined to determine whether any exceed a threshold, thereby locating the corresponding frequency of the vibration.
Once the frequency of vibration is determined, a corrective filter is configured to reduce the effect of the frequency of the vibration on the position signal. The corrective filter is configured to operate at the detected frequency of the vibration, in accordance with a plurality of filter coefficients determined as a function of the detected frequency of the vibration.
In one exemplary embodiment, the corrective filter may be of the form
Y(n)=Ap(n)+Bp(nxe2x88x921)+Cp(nxe2x88x922)xe2x88x92EY(nxe2x88x921)xe2x88x92FY(nxe2x88x922).
To ensure that the intermediate nodes of the filter operate within the allowable dynamic range of the processing, various filter structures are disclosed herein.
The filter may be used alone, or in addition to other filters, which may correct for, for example, self-vibration from the data storage device. Since the filters may only be engaged during settle-out/track follow mode, techniques for accessing the filter state vectors, as initial conditions, must be provided. In that regard, the present invention relates to, in another aspect, repeatedly engaging the filters, including repeatedly accessing filter state vectors as a function of the frequency of the vibration from the sources internal or external to the data storage device. These filter states may be refreshed periodically with corresponding outputs from the filter (in one example, about every 5 to 10 periods of the vibration). Separate counters may be maintained (M and N) to access the respective filter state vectors. The counters wrap at an index corresponding to the detected frequency of the vibration, thereby allowing the filter to access its filter state vectors, as initial conditions, as the filter is repeatedly engaged during settle-out/track-follow.
To account for changing vibration conditions, the present invention relates to, in another aspect, repeatedly detecting, during operation of the data storage device, a frequency of vibration and repeatedly configuring the corrective filter to operate at the detected frequency.
By utilizing the dual approach of first detecting, and then correcting for frequencies of vibration, and by providing the novel filter implementation structures herein, the present invention provides a solution for vibrations in data storage devices. These vibrations are often unpredictable, and the frequencies of vibration must therefore be detected and accounted for during real-time operation of the system. Further, the present invention provides low processing cost solutions to these problems, i.e., avoids FFT processing techniques, for the detection and filtering processes disclosed herein.